Custom Built Dynaco MK III - Bias Question

I have a custom-built MK III that somebody built two into a single chassis. The current tubes are in need of replacement and I need help working out the Bias current and/or voltage over a 10-ohm resistor. There is only one bias pot per side supporting two tubes. Each tube has a 10-ohm resistor. Therefore I am assuming I need to divide the 700mV bias the original MK III manual states for two tubes that share one bias resistor. Am I correct to measure the bias current flow at 35 mA a tube? I have matched quad tubes coming so hopefully each tube will be matched in the current flow. I am also assuming I can just measure the voltage drop across each tube's bias resistor at 350 mV or .35 V?

I picked up a pair of Marshall replacement (I think they are Chinese tubes) tubes that were supposed to be matched sets. However, one side was off by .05V. I never got to measure the other side. At the time I was thinking I was shooting for.7V and the tubes were hot but never red-plated. I have another set of quad-matched JJs coming and I am thinking of waiting to try them.

It was difficult because the amp is upside down so I can measure the bias resistor. Is it worth getting one of those bias tools off eBay, and what do people recommend? Some can get expensive, but when you are dealing with lethal voltage I do not want to play around.

I will post pictures once I figure out the picture hosting again.

scuba100 wrote:I have a custom-built MK III that somebody built two into a single chassis. The current tubes are in need of replacement and I need help working out the Bias current and/or voltage over a 10-ohm resistor. There is only one bias pot per side supporting two tubes. Each tube has a 10-ohm resistor. Therefore I am assuming I need to divide the 700mV bias the original MK III manual states for two tubes that share one bias resistor. Am I correct to measure the bias current flow at 35 mA a tube? I have matched quad tubes coming so hopefully each tube will be matched in the current flow. I am also assuming I can just measure the voltage drop across each tube's bias resistor at 350 mV or .35 V?

I picked up a pair of Marshall replacement (I think they are Chinese tubes) tubes that were supposed to be matched sets. However, one side was off by .05V. I never got to measure the other side. At the time I was thinking I was shooting for.7V and the tubes were hot but never red-plated. I have another set of quad-matched JJs coming and I am thinking of waiting to try them.

It was difficult because the amp is upside down so I can measure the bias resistor. Is it worth getting one of those bias tools off eBay, and what do people recommend? Some can get expensive, but when you are dealing with lethal voltage I do not want to play around.

I will post pictures once I figure out the picture hosting again.

You could solder wires from each cathode to a connector mounted at the outside
and measure the cathode voltages there.
Your calculations are right, each tube should draw app 50mA, properly matched
tubes will give same bias on both tubes in each pair.

Should I be at 50mA or go 80% for push-pull (2 tubes), so 40mA?

scuba100 wrote:Should I be at 50mA or go 80% for push-pull (2 tubes), so 40mA?

You decide. Anywhere between 40 and 50mA is ok. 40mA will have more dist, but
you might not be disturbed by it.

what is dist @peterh ?

I am playing the second set of tubes I picked up (JJ matched quad from Watford). The amp is stable at 420 - 404 mV between the quad. I will take the dance and play with fire up around 500mV. 500mV gives me the 50mA.

I have missed tube sound!!!!!!!! I have been running a very nice AA1150 solid-state amp that is very solid, but oh how tube sing.

Ok it sounds better at 50mA

Tubes have been steady at just under 50mA for a couple of hours.

soooo-I put on REM Automatic for the People and turned the volume up. Then I closed my eyes, and I am at a club with them 15" in front of me.

PeterH - Thank you for the help getting my rig back up and running. I have missed this sound! I put in test points on the top of the chaise from each tube cathode as you suggested, and what a difference in being able to get the bias set.

Thank you very much SIR!